[PATCH] tracing/function-graph-tracer: fix a regression while suspend to disk

Impact: fix a crash while kernel image restore

When the function graph tracer is running and while suspend to disk, some racyand dangerous things happen against this tracer.

The current task will save its registers including the stack pointer whichcontains the return address hooked by the tracer. But the current task willcontinue to enter other functions after that to save the memory, and thenit will store other return addresses, and finally loose the old depth whichmatches the return address saved in the old stack (during the registers saving).

So on image restore, the code will return to wrong addresses.And there are other things: on restore, the task will have it's "current"pointer overwritten during registers restoring....switching from one task toanother... That would be insane to try to trace function graphs at thesestages.This patch makes the function graph tracer listening on power events, makingit's tracing disabled for the current task (the one that performs the hibernation work)while suspend/resume to disk, making the tracing safe during hibernation.